Several laboratories have reported on engineering the fatty acid pathway in a microbial yeast such as Saccharomyces cerevisiae as a sustainable platform for producing biofuels and other products (Kalscheuer et al., 2006; Runguphan and Keasling, 2013). A range of enzymes and pathways have been heterologously expressed to convert fatty acid thioesters—produced by endogenous fatty acid biosynthesis—into ethyl esters, acids, alcohols, alkanes, methyl ketones, dicarboxylic acids, etc. (Clomburg et al., 2015; Goh et al., 2012; Zhou et al., 2016). Among these products, long-chain fatty alcohols in the C12-C18 range have recently received intense attention due to their value and broad applications in laundry detergents, industrial lubricants and surfactants, medicines and personal care products, and potentially as biofuels (Feng et al., 2015; Liu et al., 2016; Pfleger et al., 2015). In 2016, the worldwide market for fatty alcohols was $3.7 billion and growing, with annual production of more than 2.6 metric tons sourced primarily from fossil fuels (petroleum, or polymerized natural gas) or plant oil crops (triglycerides) processed chemically into alcohols (Gaikwad, 2016). Microbial production, besides providing a more sustainable source, can allow for highly specific chemical modifications (Haushalter et al., 2014) to improve product performance or create new applications in ways that could be difficult or impossible by traditional thermochemical means.
However, low yields and economic competition from mature petrochemical processes hamper widespread adoption of microbial fatty alcohol production. Traditionally, S. cerevisiae is the preferred industrial biorefinery yeast due to its genomic and structural robustness, and since existing ethanol-producing fermentation facilities could be retrofitted for another product. Yet yields of fatty alcohols in S. cerevisiae stand at less than 2% of the theoretical maximum from glucose (Zhou et al., 2016). Besides product yield, economic viability also depends on the choice of feedstocks. Sugars derived from food crops are cost-prohibitive and divert water and other resources in a period where demand for food and water is expected to increase ˜50% by the turn of this century (Connor and Uhlenbrook, 2016). Using lignocellulosic feedstocks derived from agricultural waste or energy crops that do not compete for water and land with food would lower costs and provide maximal CO2 emission offsets (Caspeta and Nielsen, 2013).